This invention relates to apparatus for causing ion beam accelerator tubes to generate neutrons and is particularly directed to apparatus of such character which are capable of being used in the logging of boreholes in the earth.
Pulsed neutron generators are known in the art, for example, those shown in U.S. Pat. No. 3,309,522 to Arthur H. Youmans, et al., and those illustrated in my U.S. Pat. No. 3,787,686. It is also known that such prior art pulsed neutron generators will emit neutrons at a steady state rate if the high voltage pulser is disabled. It is also known that such sources may exhibit pre-ignition if the pulsing frequency is much lower than 1000 cycles per second. Although this characteristic is of little or no consequence while pulsing the generator at that rate or higher, it has been put to use in the "subtraction method" of detecting gamma rays produced by inelastic scattering of fast neutrons as described in my aforementioned U.S. Pat. No. 3,787,686.
This tendency to pre-ignite or to go into a steady state conduction presents a disadvantage whenever it is desired to make logging runs which provide short half-life activation measurements, i.e., those measurements which might occur several milliseconds after the termination of the short burst of fast neutrons from the source.
It is therefore the primary object of the present invention to provide a new and improved pulsed neutron source which does not emit neutrons until an external source of excitation is applied;
It is also an object of the invention to provide a new and improved pulsed neutron source which can be extinguished upon the application of an external source of excitation; and
It is yet another object of the invention to provide a well logging system utilizing the pulsed neutron sources built in accordance with the present invention.
The objects of the invention are accomplished, generally, by the provision of a pulsed neutron source wherein a direct current path is formed between the anode and cathode of an ion beam accelerator.